Over BILLING

Guys i had contract of wifi router for almost a year and i had been paying R498 and last month they Charged me R1897 and when i called them to find out , they say i pressed continue then it started charging me from airtime which to me its new and it doesnt make sense , my contract is closed up contract, no billing what what , i only knwe that k can recharge not them billing me so now they even say i must cancel on the app so please what do i do because im confused and the customer care says who ever offered me a cobtract was supposed to explain and they didnt and i would never agree to those terms and condition and for a week i didnt have data but yet they charged me extra R1300 from normal amount and yet i couldnt connect it kept on saying out of data , if it had given me chance to connect i would mayb understand but paying for something that ididnt use . TELKOM stop this .